If he has grown taller, it could be his nipple is in the right (wrong) position now so it rubs now whereas it didnt previously. I just thought as it's just one nipple, it could be an external irritant causing the problem???

We had an issue with nipple rubbing earlier in the year with DD. It as the logo on her school tshirt. I bought her a crop top and some singlets to wear under which helped.